1. Field of technology
The present invention relates to a roll paper transportation device for conveying roll paper smoothly, and to a printing apparatus comprising this roll paper transportation device.
2. Description of Related Art
Conventional printers that use roll paper commonly have a tension-absorbing buffer mechanism that keeps the roll paper tension less than or equal to a predetermined level. This buffer mechanism is supported so that it can move elastically to maintain constant tension on the roll paper. See, for example, Japanese Unexamined Patent Appl. Pub. H06-8554.
Arrangements having a buffer plate for alleviating the high tension produced by the inertia of the core around which the paper is rolled at the start of roll paper transportation are also known. Inertia increases as the diameter of the paper core increases. Compared with a printer in which the roll paper is supported on a shaft, the inertia of the core is particularly great and the resulting tension is high in so-called drop-in loading printers in which the paper roll is simply placed inside the roll paper compartment.
A paper detection unit may also be disposed near the buffer plate to detect if roll paper is present. Friction produced during roll paper transportation can produce static electricity, which can build up and then be discharged to the paper detection unit near the paper transportation path, thus damaging or destroying the paper detection unit. If the paper detection unit uses a photosensor, the location of the circuit unit at the sensor surface makes the circuitry particularly susceptible to damage caused by the static charge being discharged directly to the circuit unit. A metal buffer plate can be disposed surrounding the photosensor in order to avoid static discharge by draining the electrostatic charge through the metal buffer plate, thereby preventing discharge to the photosensor as taught, for example, in Japanese Unexamined Patent Appl. Pub. 2000-62281. This, however, requires a complicated metal processing operation to shape the buffer plate, thus increasing the cost. In addition, the buffer plate must be electrically connected to the frame by a wire, for example, in order to reliably drain the static charge from the buffer plate to the frame, thus further complicating the construction and adding to the cost.
An arrangement that uses a mechanical switch could be used instead of a photosensor in the paper detection unit as a means of avoiding damage caused by static electricity. FIG. 7 is a section view showing the paper detection unit and buffer mechanism in a conventional thermal printer. As shown in FIG. 7 this printer 30 holds a paper roll 32 into which the roll paper 31 is wound. The roll paper 31 is held between the print head 33 and platen 34. Rotating the platen 34 causes the roll paper 31 to advance. The paper detection unit 37 detects the presence of roll paper 31 in response to movement in the position of a switch lever 38 extending from a paper detection unit 37. The switch lever 38 pivots to separate the paper detection unit 37 from the roll paper 31. In addition, the switch lever can be made from plastic or other nonconductive material so that static electricity produced by the roll paper 31 can be prevented from being discharged to the paper detection unit 37. The buffer plate 36 of the buffer mechanism 35 and the switch lever 38 of the paper detection unit 37 are also disposed to the transportation path between the paper roll 32 and platen 34 along the transportation direction of the roll paper 31. The buffer plate 36 can rock and thus works to absorb the strong tension applied to the roll paper 31 by the inertia of the paper roll 32 when paper transportation starts.
The paper detection unit 37 for detecting the roll paper 31 is composed of switch lever 38 and a mechanical switch 39. The switch lever 38 touches the roll paper 31 and can pivot. If roll paper 31 is present, the switch lever 38 pivots and moves to position Q, causing the contact part 41 on the end of the switch lever 38 to contact the contact lever 40 (contact terminal) inside the mechanical switch 39, and thus turns the switch on. If roll paper 31 is not present, the switch lever 38 moves to position P, thus separating from the contact lever 40 and turning the switch off. When roll paper 31 is loaded and contacts the switch lever 38, the switch lever 38 pivots and moves to position Q, the contact part 41 contacts contact lever 40, and the switch turns on. Whether roll paper 31 is present can thus be detected from the change in the on/off state of the switch.
A problem with the prior art taught in Japanese Unexamined Patent Appl. Pub. H06-8554 is that the buffer mechanism requires the installation space between the paper roll 32 and platen 34 to be large to accommodate both the buffer mechanism and paper detection apparatus. As shown in FIG. 7, a long distance L1 is required between the paper roll 32 and platen 34 in order to dispose the buffer mechanism 35 and paper detection unit 37 along the transportation direction of the roll paper 31.
Yet further, when a mechanical switch is used instead of a photosensor for the paper detection unit in order to avoid damage caused by electrostatic discharge, problems specific to the mechanical switch also arise. When the roll paper 31 is conveyed, variation in the load due to the friction of roll paper transportation, as well as shifting of the roll paper side to side and variation in the vertical movement of the paper during roll paper transportation, cause the position of the switch to change. Though slight, the roll paper 31 position also changes repeatedly between roll paper positions R1 and R2 even when a buffer plate 36 is used. As the position of the roll paper 31 changes, the switch lever 38 also pivots repeatedly, although the distance moved is slight, and the switch lever 38 thus slides repeatedly in contact with the contact lever 40 (in the on position) at the contact part 41 of the mechanical switch 39. This gradually wears on the contact part 41 and eventually results in poor electrical contact.
To solve these problems, an object of the present invention is to provide a roll paper transportation device and printing apparatus that minimizes the space required to accommodate the buffer mechanism and paper detection unit and to prevent poor electrical contact at the contact part of the mechanical switch that is used in the paper detection unit.